384 



ORDOVICIAN PERIOD 



and the starfish and sea-urchin types had made their appearance. 

 Thecystoids (a,b andc, Fig. 342), with their irregular forms, were the 

 most primitive, and gave place in time to the more symmetrical cri- 

 noids (Fig. 342, d to k], which may be likened to star-fishes turned 



J ^JP ft ^ 8T \ 



Fig. 342. ORDOVICIAN ECHTNODERMS: a, Comarocystis punctatus Billings; b, 

 Lcpidodiscus cincinnaticnsis (Roemer); c, Plcurocystis filitcxtus Billings; d, Ectcno- 

 crinus grandis (Hall) ; e, Dendrocrinus polydactylus (Shumard) ;/, Hybocrinus tumidus 

 Billings; g, Lepadocystis moorci (Meek); h, Carabocrinus vancortlandti Billings; i, 

 Archaocrinus desidcratus Billings; j, Glyptocrinus decadactylus Hall; k, Anomalo- 

 crinus incurvus M. and W.; /, Palceaster simplex Miller, a, b and c are cystoids, d-k 

 are crinoids, / is a star-fish. 



face uppermost and fixed to the sea-bottom by a calcareous stem 

 attached to the center of the back. Crinoids so closely re- 

 sembled a flower in form, that the familiar name "sea-lily" is not 

 inappropriate. 



Corals are few in the lower part of the system, and though more 

 abundant in higher beds, are nowhere a leading part of the fauna. 



